Cu sites in biology are involved in a wide variety of functions including O2 binding, activation for hydroxylation and cofactor formation, the four electron reduction of O2 to H2O and the two electron reduction of N2O to N2. These often exhibit unique spectroscopic features that reflect novel geometric and electronic structures that make major contributions to function. Research involves the application of a wide variety of spectroscopic (absorption, variable-temperature variable-field magnetic circular dichroism, resonance Raman, EPR, SQUID magnetic susceptibility, X-ray absorption, etc.) and electronic structure methods (ligand field and density functional theory, etc.) to define the active sites in these proteins, related model complexes and their intermediates and determine geometric and electronic structure contributions to function. Specific aims are:1) Definition of the reaction coordinate of oxy-tyrosinase and determination of differences in substrate interaction with the similar coupled binuclear copper sites in hemocyanin, tyrosinase, mutants of tyrosinase associated with oculocutaneous albinism, and catechol oxidase which relate to differences in function; 2) Definition of the coordinatively unsaturated nature of the trinuclear Cu cluster site in the multicopper oxidases, its reaction coordinate for O2 reduction to H2O, its coupling with the Type 1 center as related to intramolecular electron transfer and its interaction with metal ion substrates relevant to iron metabolism in ceruloplasmin and its genetic disorder aceruloplasminemia, Fe uptake in yeast (Fet3p) and microbial Mn and Cu oxidation; 3) Extension of Cu(II)-hydroperoxide model studies to the active sites in dopamine (3- monooxygenase and peptidylglycine a-hydroxylating monooxygenase, involved in the control of neurotransmitters and peptidic hormone production, to determine the reaction coordinate for O2 activation by a single Cu center; 4) Definition of electronic structure/reactivity correlations for the m-sulfide bridged tetranuclear Cuz cluster which catalyzes the two electron reduction of the green house gas N2O; 5) Determination of the nature of tyrosine residue activation by Cu(II) sites in the O2 dependent biosynthesis of the organic cofactors in amine oxidase and galactose oxidase; 6) Definition of O2 intermediates in heme/Cu models of cytochrome c oxidase, the terminal enzyme in aerobic metabolism, to understand its reaction coordinate for O2 reduction relative to the multicopper oxidases and how this relates to proton pumpingfor ATP synthesis.